U.S. patent number 5,810,705 [Application Number 08/704,349] was granted by the patent office on 1998-09-22 for developing roller.
This patent grant is currently assigned to Kanegafuchi Kagaku Kogyo Kabushiki Kaisha. Invention is credited to Shigeo Akimoto, Susumu Fukuda, Kenji Kobayashi, Yasunori Matsunari, Kazuyoshi Mimura, Yoshihisa Tawada.
United States Patent |
5,810,705 |
Mimura , et al. |
September 22, 1998 |
Developing roller
Abstract
A developing apparatus of a toner delivery system by contacting
a developer roller with an electrostatic latent image-carrying
member eliminates a protective layer in the conventional concept
from a surface of the developing roller, and provides a covering
layer which is formed by adhering particles of toner or a material
having electrical characteristics and mechanical characteristics
equal to those of the toner on the surface of the developing
roller. Those particles are adhered utilizing a surface tackiness
of an elastomer layer itself which is a structural element of the
developing roller.
Inventors: |
Mimura; Kazuyoshi (Otsu,
JP), Tawada; Yoshihisa (Otsu, JP), Akimoto;
Shigeo (Otsu, JP), Kobayashi; Kenji (Otsu,
JP), Matsunari; Yasunori (Otsu, JP),
Fukuda; Susumu (Otsu, JP) |
Assignee: |
Kanegafuchi Kagaku Kogyo Kabushiki
Kaisha (Osaka, JP)
|
Family
ID: |
24829111 |
Appl.
No.: |
08/704,349 |
Filed: |
August 28, 1996 |
Current U.S.
Class: |
492/56;
492/59 |
Current CPC
Class: |
G03G
15/0818 (20130101); G03G 2215/0861 (20130101) |
Current International
Class: |
G03G
15/08 (20060101); B23P 015/00 () |
Field of
Search: |
;492/56,59 ;399/333,321
;428/36.91 |
References Cited
[Referenced By]
U.S. Patent Documents
|
|
|
5386279 |
January 1995 |
Fukami et al. |
5458937 |
October 1995 |
Nakamura et al. |
5581329 |
December 1996 |
Kosmider et al. |
5582917 |
December 1996 |
Chen et al. |
5640662 |
June 1997 |
Sugimoto et al. |
5659869 |
August 1997 |
Ohtsuka et al. |
|
Primary Examiner: Cuda; Irene
Attorney, Agent or Firm: Armstrong, Westerman, Hattori,
McLeland & Naughton
Claims
What is claimed is:
1. A developing roller which acts to deliver a toner to a surface
of an electrostatic latent image-carrying member having an
electrostatic latent image formed thereon, comprising
a conductive axial body,
an elastomer layer which is formed from a resin composition having
a surface tackiness and also having a conductivity or a
semiconductivity, comprising a reactive organic material, formed on
a surface of the axial body, and
a covering layer as an outermost layer, which is formed by adhering
a toner on the surface of the elastomer layer utilizing the surface
tackiness of the elastomer layer, thereby covering the surface of
the elastomer layer with the toner.
2. A developing roller which acts to deliver a toner to a surface
of an electrostatic latent image-carrying member having an
electrostatic latent image formed thereon, comprising
a conductive axial body,
an elastomer layer which is formed from a resin composition having
a surface tackiness and also having a conductivity or a
semiconductivity comprising a reactive organic material, formed on
a surface of the axial body, and
a covering layer as an outermost layer, which is formed by adhering
a material having substantially the same electrical and mechanical
characteristics as the toner on the surface of the elastomer layer
utilizing the surface tackiness of the elastomer layer, thereby
covering the surface of the elastomer layer with the material.
3. The developing roller as claimed in claim 1, wherein the surface
tackiness of the elastomer layer is in a range of 1 to 13 in an
inclined ball tack test (inclination 30.degree., 23.degree. C.)
according to JIS Z 0237.
4. The developing roller as claimed in claim 3, wherein (the toner)
which forms the covering layer has a nearly spherical shape.
5. The developing roller as claimed in claim 3, wherein the resin
composition which is the constituent material of the elastomer
layer is oxyalkylene type, saturated hydrocarbon type, urethane
type or siloxane type resin composition, and is a curable resin
composition comprising a reactive organic material which becomes
from a liquid to a solid by the curing reaction.
6. The developing roller as claimed in claim 5, wherein the curable
resin composition is the oxyalkylene type resin composition, and
the curing reaction is hydrosilylation reaction.
7. The developing roller as claimed in claim 5, wherein the curable
resin composition is the saturated hydrocarbon type resin
composition, and the curing reaction is hydrosilylation
reaction.
8. The developing roller as claimed in claim 2, wherein the surface
tackiness of the elastomer layer is in a range of 1 to 13 in an
inclined ball tack test (inclination 30.degree. , 23.degree. C.)
according to JIS Z 0237.
9. The developing roller as claimed in claim 8, wherein the
material having substantially the same electrical and mechanical
characteristics as the toner which forms the covering layer has a
nearly spherical shape.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a developing roller which is used
in a developing unit incorporated in an apparatus employing an
electrophotographic system, a back exposure system (so-called a
chargeless system) or an electrostatic recording system, such as a
printer, a copying machine or a receiver in a facsimile. More
particularly, the present invention intends to provide the
developing roller that exhibits the most remarkable effect when
used in an apparatus employing a one-component non-magnetic
development system.
2. Description of the Related Art
Electrophotographic systems include a system utilizing both a
magnetic working force and an electric working force for delivery
of a developer, and a system utilizing only the electric working
force for delivery of a developer. The system utilizing both the
magnetic working force and the electric working force also includes
a system using a two-component developer comprising a magnetic
carrier and a non-magnetic toner, and a system using a
one-component developer consisting of a magnetic developer. To the
contrary, the system utilizing only the electric working force
includes a system using the one-component developer consisting of a
non-magnetic toner (hereinafter referred to as "one-component
non-magnetic developer).
FIG. 4 shows a developing unit in a printer or a copying machine
using the one-component non-magnetic developer, and shows the
peripheral structure thereof. This structure is first explained
below.
A photoconductive insulative layer is formed on the surface of a
photosensitive member (drum) 1 which is an electrostatic latent
image-carrying member. The photoconductive insulating layer
consists of a material which generally shows an electrically
insulating property and shows a conductivity when irradiated with
light. The photosensitive member (drum) 1 rotates in a rotating
direction 1a, and the surface thereof is electrified uniformly with
a charging device 2. The surface of the photosensitive member
(drum) 1 is irradiated with a light irradiation 10 of a reflected
light when scanning the surface of an original by light, through an
optical system (not shown), and the areas which receive a light
irradiation on the surface of the photosensitive member 1 become
conductive, thereby disappearing electric charges on the areas. As
a result, electrostatic latent images are formed on the surface of
the photosensitive member 1. The light irradiation 10 is conducted
with a light beam having an intensity modulated corresponding to an
image to be recorded in, for example, a laser printer. In a copying
machine, the light irradiation 10 is performed with a light derived
from the reflected light when scanning the surface of an original
by a light through an optical system. When a toner which is an
electrified powdery one-component non-magnetic developer is coated
from a developing roller 33 to a latent image formed on the surface
of the photosensitive member 1, the latent image is manifested.
A toner supplied on the surface of the photosensitive member 1 is
transferred onto a recording paper 4. In performing this transfer,
an electrostatic suction force is acted from a back side of the
recording paper 4 with a transfer machine 5. A cleaner 6 such as a
cleaning blade is arranged at a downstream side of the transfer
machine 5 in the rotating direction 1a. This cleaner 6 removes
toners which are adsorbed on the surface of the photosensitive
member 1 but are not transferred onto the recording paper 4.
The recording paper 4 having toners transferred thereonto is
carried to a fixing apparatus 7. The fixing apparatus 7 comprises a
heating roller 8 and a pressure roller 9. The recording paper 4 is
passed through therebetween to thereby fix the transferred toners
onto the recording paper 4.
A developing apparatus 3 has a structure that a developing roller
33 is built in a toner reservoir 31 in which a toner 12 is stored.
The photosensitive member 1 is in contact with the surface of the
developing roller 33 to coat the toner 12 on the surface of the
photosensitive member 1. This type of development method is called
a contact method. In the toner reservoir 31, a toner supplying
roller 34 for sufficiently carrying the toner 12 onto the surface
of the developing roller 33 and a thin layer-forming blade 36 for
controlling a layer thickness of the toner on the surface of the
developing roller 33 are arranged in contact with the developing
roller 33.
A voltage which is negative to a ground potential is applied to the
developing roller 33 through a negative electric source 37, and a
voltage which is negative to the ground potential, larger than the
voltage applied to the developing roller 33 in terms of an absolute
value is applied to the toner supplying roller 34 through a
negative electric source 38, thereby electrifying the toners
adhered to, and deposited on the surface of the developing roller
33.
It should be noted that the cleaner used may be a brush or a
roller, besides the blade system as shown in the drawing.
The contact type developing apparatus using the one-component
non-magnetic developer and the peripheral structure thereof are as
described above. However, the conventional developing roller used
in the apparatus has the following disadvantages.
Since the developing roller contacts with the photosensitive
member, it is required to have a softness to an extent that the
photosensitive member is not damaged. Also, it is necessary that a
contact width to the photosensitive member in a circumferential
direction, i.e., a nip width between the developing roller and the
photosensitive member, be about 0.5 to 2 mm. To this effect, it is
preferred that the developing roller has a hardness of 8 to
30.degree. as defined in JIS Hardness A.
Materials used for the developing roller include a
nitrile-butadiene rubber (NBR), an ethylene-propylene-diene
copolymer (EPDM), silicones, urethanes, and the like. Single use of
NBR, EPDM or urethanes gives too large hardness. Therefore, if
those materials are used as the developing roller, it is necessary
to add a plasticizer to those materials in order to decrease the
hardness. Further, a vulcanizing agent is generally added to NBR or
EPDM in order to accelerate crosslinking. However, addition of such
a vulcanizing agent and a plasticizer may contaminate the
photosensitive member.
Regarding silicones, low molecular weight silicones tend to bleed
on the surface, and therefore there is a fear that the
photosensitive member may be contaminated.
Thus, in either case, there is a risk to contaminate the
photosensitive member. For that reason, a protective layer is
formed on the surface of the developing roller in order to prevent
the photosensitive member from contamination. However, if a
protective layer made of a resin is formed on the surface of the
developing roller, the production steps increase. In addition, it
is necessary to select a material for the protective layer, which
does not adversely affect the photosensitive member and the toner,
although this selection is not easy. Those elements result in
increasing the production cost.
Another important problem is that charges given to the toner by the
developing roller are not stable and also not constant. As a
result, an image obtained is not stabilized. This is explained
below.
In general, charges are applied to the toners by both
triboelectrification and electrification with charges in an
electric field. The triboelectrification is performed by the
friction of the toner being contacted with the peripheral members
until the toner in the toner reservoir is adhered onto the surface
of the photosensitive member through the contact with the surface
of the developing roller. Charging by applying electric field is
performed by passing through the toner between the toner supplying
roller and the developing roller, and between the blade and the
developing roller to which electric field is applied. The toner may
contain a charge control agent or the surface of the toner may be
coated with a charge control agent, so that a predetermined
polarity or predetermined amount of charges is imparted by
friction.
Here, the important points in the electrification of the toner are
that a polarity of the toner esterified is a predetermined uniform
polarity, and the amount of electrification is fallen within the
predetermined range. However, control of imparting charges by
electric field is relatively easy, but the phenomenon of
triboelectrification which is another element in imparting charges
is a very unstable phenomenon. The triboelectrification is easily
influenced by press-contacting force of each member to the
developing roller and other environment, particularly humidity, and
it is difficult to maintain a predetermined amount of charges.
Further, since the triboelectrification amount varies by the change
of material characteristics of the surface of the developing
roller, the degree of adhesion of the toner, and the like, it is
extremely difficult to always impart charges to the toner in a
stable manner.
In order to control the triboelectrification, it is necessary to
design the entire developing apparatus, sufficiently considering
the triboelectrification rank of members to which the toner slides
and contacts until the toner transfers from the toner reservoir
onto the surface of the photosensitive member, but such a design is
not easy. If the electrification is unstable, the electrification
amount of each toner scatters, and in an extreme case, the toner
having an opposite polarity may be present, resulting in remarkable
decrease in image quality.
Further, there is a problem on the control of resistance value of
the developing roller. It is required for the developing roller to
have a resistance value in the range of about 10.sup.3 to 10.sup.8
.OMEGA., but it is difficult to stably obtain the resistance value
in such a range. In order to realize the resistance value in such a
range, it is necessary to impart conductivity to the material of
the developing roller in any fashion, and to adjust the roller
resistance value. However, since the resistance in such a range has
the tendency to greatly vary with a delicate change in the amount
of the conductivity-imparting agent to be added, it is difficult to
control the roller resistance value to obtain the developing roller
having less scatter in resistance value.
Furthermore, there is a problem involved in colorization. In a
color system, a plurality of color toners (C, Y, M, B) are used,
and a plurality of developing rollers are used corresponding to
those toners having a plurality of colors. Each of those color
toners has a different electrification characteristic due to
difference in the respective colored pigment. Therefore, it is
difficult to conduct a stable color development, and matching
between the developing roller and the development system is
difficult.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above
circumstances, and an object of the present invention is to provide
a developing roller which can stabilize the electrification
characteristic of a toner and can attempt a high quality of an
image.
Another object of the present invention is to provide a developing
roller which can eliminate the occurrence of cost accompanied with
the formation of a protective layer on the surface of the
developing roller, and can cope with a colorization.
As a result of various investigations to overcome the
above-described problems involved in the conventional technique, it
has been found that the problems can be overcome by forming a
protective layer that constitutes a covering layer of the
developing roller with a developing toner itself or from a material
having substantially same electrical characteristics and mechanical
characteristics as those in the toner. It has also be found that
such a constitution makes it possible to greatly reduce factors
that unstabilize the amount of charges imparted to the toner,
thereby being capable of substantially uniformly controlling
charges imparted to the toner. Further, it has been found that the
covering layer may be formed without any cost if the formation of
the covering layer comprising those materials can be realized by
adhering those materials utilizing surface tackiness of a
conductive elastomer layer of the developing roller.
The present invention has been completed based on those
findings.
According to the present invention, there is provided a developing
roller which acts to deliver a toner to the surface of an
electrostatic latent image-carrying member having an electrostatic
latent image formed thereon, comprising
a conductive axial body,
an elastomer layer which is formed of a resin composition having a
surface tackiness and also having a conductivity or a
semiconductivity comprising a reactive organic material, formed on
the surface of the axial body, and
a covering layer as an outermost layer, which is formed by adhering
a toner onto the surface of the elastomer layer utilizing the
surface tackiness of the elastomer layer, thereby covering the
surface of the elastomer layer with the toner.
In order to form the covering layer on the surface of the elastomer
layer, a material to be adhered utilizing the surface tackiness
needs not be the toner itself, and any material may be used if it
has electrical characteristics and mechanical characteristics
equivalent to those of the toner.
It can generally be understood that the term "equivalent electrical
characteristics and mechanical characteristics" used herein means
that a triboelectrification rank is equivalent. The
triboelectrification rank is arranged based on the experiential
rule such that when two substances are contacted, a substance which
is positively electrified is positioned upward, and a substance
which is negatively electrified is positioned downward. However,
since this is not strictly prescribed, the rank may be replaced in
similar substances with each other.
The surface tackiness of the elastomer layer is defined in an
inclined ball tack test (inclination 30.degree., 23.degree. C.)
according to JIS Z 0237, and a material having a surface tackiness
of 1 to 13 can be used. The inclined ball tack test is explained in
detail hereinafter.
It is preferred that the toner for forming the covering layer has a
nearly spherical shape from the point of clearness (definition) of
the image.
The resin composition which is the constituent material of the
elastomer layer comprises an oxyalkylene type, a saturated
hydrocarbon type, an urethane type, or a siloxane type as a main
component, which is a reactive organic material that converts from
a liquid into a solid by curing reaction. Of those, the oxyalkylene
type and the saturated hydrocarbon type are preferred.
The developing roller according to the present invention has a
structure in which the toner itself which constitutes the covering
layer of the developing roller or a material having a
triboelectrification rank equivalent to that of the toner is
adhered onto the surface of the elastomer layer by the surface
tackiness of the resin composition which constitutes the elastomer
layer. This covering layer prevents the resin composition from
being directly contacted with the photosensitive member, and also
functions as a protective layer to prevent from bleeding.
The toner is supplied to the covering layer of the developing
roller from a toner supplying roller arranged in a toner reservoir.
A material which constitutes the covering layer of the developing
roller is the toner itself, or a material having electrical
characteristics and mechanical characteristics equivalent to those
of the toner, so that additional electrification by friction with
the surface of the developing roller does not substantially occur,
and imparting charges to the toner is substantially controlled by
electric field generated by voltage applied to the developing
roller, the toner supplying roller or a blade.
It is easy to control voltages applied to the developing roller or
the toner supplying roller, so that electrification of developer
particles is conducted stably, and electrification polarity of the
toner and the amount of electrification are substantially uniform.
As a result, a stabilized image can be obtained.
The developing roller of the present invention is suitable for
color development. When developing roller of the present invention
is applied to a developing apparatus of color system, the
electrification characteristics of each color toner (C, Y, M, B)
differ, but the surface of the developing roller provided
corresponding to each color toner is covered with the same color
toner or a material having electrical characteristics and
mechanical characteristics equivalent to those of the same color
toner. Therefore, color matching can be conducted with only control
of electric field, similar to the case of monochrome, and a color
development can be conducted in a stable manner.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an explanatory view showing a cross sectional structure
of the developing roller;
FIG. 2 is an explanatory view showing a state that a toner for
development is deposited on the surface of the developing roller in
the working state;
FIG. 3 is an explanatory view showing a test method of a inclined
ball tack test; and
FIG. 4 is an explanatory view showing one embodiment of the
developing apparatus and its peripheral structure.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Next, the present invention is explained in detail with reference
to the accompanying drawings. In the explanation described below,
the present invention is explained by referring to the embodiment
wherein a one-component non-magnetic developer is used as a
developer, but the present invention is not limited to this
embodiment.
The developing apparatus having the developing roller according to
the present invention incorporated therein and the peripheral
structure thereof can basically employ the conventional ones except
for the structure of the developing roller 33 itself. For example,
the developing apparatus and the peripheral structure thereof
similar to those as shown in FIG. 4 can be exemplified.
The developing apparatus 3 for coating the toner on the surface of
the photosensitive member (drum) 1 has such a structure that the
developing roller 33 and the toner supplying roller 34 are
incorporated in the toner reservoir 31 filled with the powdery
toner 12. The toner is formed by covering colored pigment with a
thermoplastic resin such as a styrene acrylic resin or a polyester
resin. The powdery toner has a particle size of about 10 .mu.m. The
toner supplying roller 34 acts to efficiently supply the toner on
the surface of the developing roller 33. The arrangement of this
toner supplying roller 34 makes it possible to smoothly transfer
the toner 12 to the developing roller 33, and also makes it
possible to impart electric charges to the toner. In this
embodiment, DC voltage of -150 to -350 V is applied to the
developing roller 33 from the electric source 37, and DC voltage of
-200 to -600 V is applied to the toner supplying roller 34 from the
electric source 38.
The toner supplying roller 34 which can be used is a roller made of
a sponge-like material such as a conductive foamed material, or a
conductive cylindrical material such as a metal pipe. As specific
examples of the material for the roller, urethane, aluminum, and
the like are exemplified.
On the outer surface of the developing roller 33, a blade 36 as a
means for controlling the thickness of the toner layer is provided.
The edge of the blade is contacted with the surface of the
developing roller 33 having the developing toner adhered thereonto
to regulate the amount of the toner deposited on the surface of the
developing roller 33. In this embodiment, the direction of the edge
of the blade is opposite to a rotating direction 33a of the
developing roller 33, but the direction of the edge of the blade is
not particularly limited. A means for applying direct current
voltage is arranged on the blade in order to impart predetermined
charges to the toner between the blade and the developing roller,
and voltage of -150 to -600 V is applied. It is preferred that the
blade 36 be formed using a conductive material. The material for
the blade 36 preferably has a hardness higher than the hardness of
the material for the developing roller 33.
The above embodiment in which voltage is applied to the developing
roller, the toner supplying roller and the blade is directed to the
use of negatively charged toner, and when positively charged toner
is used, the voltage to be applied has an opposite polarity. In
either use of negatively charged toner or positively charged toner,
AC voltage may be superimposed on DC voltage.
FIG. 1 is a sectional explanatory view showing a schematic
structure of the developing roller 33. The developing roller 33 has
a structure that an elastomer layer 41 made of a resin composition
having a surface tackiness is formed on the circumference of a
conductive axial body 40 such as a metal shaft and a covering layer
42 comprising a toner or the similar material is formed on the
surface of the layer. In the embodiment shown in the drawing, the
elastomer layer 41 is a monolayer, but the elastomer layer 41 may
has a structure that a plurality of the elastomer layers having
different hardness are laminated to adjust the conductivity and the
hardness of the entire developing roller 33.
The metal shaft which can be used in the present invention is not
particularly limited, and the shafts which are generally used as
the metal shaft for the developing roller, such as iron shaft,
stainless steel shaft or shaft plated with nickel or chromium, can
be used.
The thickness and the hardness of the conductive composition which
covers the metal shaft are also not particularly limited, and the
conventionally used conductive composition having a thickness of 2
to 8 mm, and preferably 3 to 5 mm, and a hardness of 5.degree. to
80.degree. defined in JIS Hardness A may be used. In particular,
the composition having a hardness about 8.degree. to 30.degree. of
JIS Hardness A is preferred from the standpoint of preventing the
blade and the photosensitive member (drum) from damage. The JIS
Hardness A measured herein is a value obtained by producing a
cylindrical sample having a diameter of 30 mm and a height of 12.7
mm with no shaft, and then measuring at 23.degree. C.
The resin composition which constitutes the elastomer layer 41 is a
composition having a surface tackiness, and includes composition
having a conductivity or a semi-conductivity. The term
"conductivity" used herein means a composition having a
conductivity of 10.sup.6 .OMEGA..multidot.cm or less, and the term
"semi-conductivity" used herein means a composition having a
conductivity of 10.sup.6 to 10.sup.10 .OMEGA..multidot.cm.
The conductivity of the elastomer layer on the developing roller is
set such that a resistance value of the entire roller is about
10.sup.3 to 10.sup.8 .OMEGA., and preferably 10.sup.4 to 10.sup.7
.OMEGA.. Expressing this with a volume resistivity of the resin
composition which forms the elastomer layer, the usable range is
10.sup.3 to 10.sup.10 .OMEGA..multidot.cm, and preferably 10.sup.5
to 10.sup.8 .OMEGA..multidot.cm. It is preferred that the volume
resistivity of the elastomer layer is set to be lower than that of
the toner or the equivalent material. The roller resistance value
described herein is a value. measured by applying a load of 500 g
to both edges of the conductive axial body and applying a direct
current voltage of 100 V thereto.
The resistance value of the developing roller can be uniformly
controlled in the range of about 10.sup.3 to 10.sup.8 .OMEGA., and
preferably 10.sup.4 to 10.sup.7 .OMEGA., by setting the thickness
of the covering layer so that the volume resistivity of the
elastomer layer is set to satisfy the above-mentioned
condition.
As a method for imparting the conductivity to the resin, addition
of conductivity-imparting substances such as carbon black or metal
powder is generally known. Addition of the conductivity-imparting
substances is not always necessary at the high resistance side in
the semiconductivity.
The covering layer 42 is formed of the toner or the equivalent
material, and the toner or the equivalent material is shown by
white particles in the drawing. The toner or the equivalent
material which constitutes the covering layer 42 is adhered
utilizing the surface tackiness possessed by the elastomer layer
41. The surface tackiness that the elastomer layer 41 possesses is
an inherent property of the material. The degree of the surface
tackiness of the elastomer layer 41 is 1 to 13, preferably 2 to 13,
more preferably 2 to 7, and most preferably 2 to 5, by an inclined
ball tack test (inclination 30.degree., 23.degree. C.) according to
JIS Z 0237.
Here, the inclined ball tack test is briefly explained
hereunder.
FIG. 3 shows a ball rolling device used in the inclined ball tack
test. The ball rolling device has a structure that a test piece 51
is adhered onto the entire length of a inclined plane inclined with
a certain inclination to a horizontal plane, with facing the
adhesive surface upward, a film is adhered onto a predetermined
area from the upper side of the inclined face to form a runway 52,
and an exposed portion of the test piece 51 positioned downward
from the runway 52 constitutes a measurement portion.
A plurality of balls (steel balls) 53 having different sizes are
successively rolled from the top edge of the runway 52, and it is
confirmed whether or not those steel balls 53 stop on the way of
the measurement portion. The degree of surface tackiness of the
test piece 51 is expressed by the number of the balls that
stopped.
Each element which constitutes the ball rolling device is explained
below.
Inclined Plate:
The inclined plate used is a smooth and rigid plate (such as glass
plate, metal plate, wood plate or plastic plate). The degree of
inclination is selected from 20.degree., 30.degree. and 40.degree..
The test results also show this degree of inclination.
Runway:
Runway is formed by adhering a transparent film having a length of
100 mm or more and a thickness of 25 .mu.m as defined in JIS C 2318
onto the tacky surface of the test piece at a predetermined
position. The length of the runway is 100 mm.
Measurement Portion:
The measurement portion is a tacky face positioned in a range of
100 mm from the bottom edge of the runway.
Ball (steel Ball):
As a material of the ball, a high carbon chromium shaft
ball-bearing steel defined in JIS G 4805 is used. Sizes of the ball
are 31 kinds of sizes of 1/16 to 1 in "ball designation" as defined
in JIS B 1501 except for 5/64, 7/64, 9/64, 15/64 and 17/64.
Using this ball rolling device, test is conducted with the
following procedures (1) to (6).
(1) The test device is fixed horizontally onto a measurement stand
using a level. An angle of an inclined plate is 30.degree.. If
necessary, an angle of 20.degree. or 40.degree. may be used.
(2) A test piece is set, facing a tacky surface upward, at a
predetermined position of the inclined plate using an adhesive tape
for fixing the upper edge of the test piece, an adhesive tape for
fixing the lower edge of the test piece, or a sash weight (mass:
about 500 g), and a polyester film for a runway is adhered to a
predetermined position on the tacky surface of the test piece.
(3) The starting position of the ball is adjusted in conformity
with a size of the ball so that the length of runway is constant,
100 mm.
(4) The boll, the surface of which being cleaned, is placed on the
starting position, and the ball is rolled.
(5) A series of procedures of adjustment of the starting position
of the ball, and rolling of ball are repeated changing the size of
the ball, and of the balls which stop within the measurement
portion (i.e., the ball does not move for 5 seconds or more), a
ball having the largest size is found. Three balls, one having the
largest size found by the same test piece, the remaining two balls
having a size next larger or next smaller than the largest size,
each is rolled three times, and it is confirmed that the ball found
is fallen within the measurement rule.
(6) Numerical value of 32 times of "ball designation" as defined in
JIS B 1501 is called ball number. The test result is expressed by
the ball number of the ball found having the largest size, and an
average value of three test pieces is shown as the test result. In
addition, the angle of the inclined plate is also indicated in the
test result.
The surface tackiness of the elastomer layer is measured by this
inclined ball tack test.
If the surface tackiness decreases, the toner which becomes the
covering layer is difficult to uniformly adhere. As a result, a
stable electrification to the toner which is a developer cannot be
conducted, and the amount of the toner delivered does not
stabilize, thereby causing an unevenness of image density and a
fogging.
On the other hand, if the surface tackiness increases too much, in
forming the elastomer layer, when the resin composition is poured
into a mold to harden it, and the molded product is picked up
thereafter, a pick-up property becomes poor. As a result, scratches
may be formed on the surface of the molded product, or the life of
the mold may be shortened.
In the case that the tackiness is 1 to 13, and preferably 2 to 13,
the covering layer can easily be formed by adhering the
one-component non-magnetic developer onto the surface of the
roller, and the electrification of the one-component non-magnetic
developer used as the toner and the control of the amount thereof
can appropriately be conducted, so that it is preferable. In the
case that development is conducted using a developing roller having
the one-component non-magnetic developer adhered uniformly onto the
surface of the roller, good image quality can be obtained.
Furthermore, in the case that the tackiness is 2 to 7, and
particularly 2 to 5, the life of the developing roller can be
prolonged (evaluated by the image quality when developed 1,000
sheets). If the tackiness exceeds 13, it is difficult to conduct
the development for a long period of time in a stable manner, and
on the other hand, if the tackiness is less than 1, adhesion of the
toner is insufficient, and the unevenness of image density or the
fogging may occur as described above.
JIS Hardness A used for expressing hardness of the elastomer layer
in the present invention is briefly explained below.
When a test piece having a thickness of 12 mm or more is mounted on
a spring type hardness tester Model A, and a pressurized face of
the tester is contacted with the test piece, a pushing needle
projected from a hole formed at the center of the pressurized face
by spring pressure is pushed back by a rubber face, and this
distance is measured. JIS Hardness A shows this distance as a
numerical value showing the hardness.
FIG. 2 shows the developing roller 33 in the driving state, and
shows the state that toners T (expressed by black dots in the
drawing) are deposited on the outer surface of the covering layer
42 formed on the surface of the elastomer layer 41. According to
the developing roller of the present invention, since the covering
layer 42 as the outermost layer thereof is formed of the toner
itself or a material equivalent to the toner, a material with which
the toner T for development supplied on the surface of the
developing roller 33 contacts is the toner itself or a material
equivalent to the toner. Therefore, it is difficult for the toner
for development which has been deposited to cause the
triboelectrification, and the change in the amount of
electrification rarely occurs.
The material equivalent to the toner, which constitutes the
covering layer preferably has the electrical characteristics and
mechanical characteristics close to those of the toner in order to
exclude the triboelectrification as much as possible, and therefore
it is preferable to select a material, for example, having a close
triboelectrification rank. Specifically, a material having a volume
resistivity of 10.sup.6 to 10.sup.14 .OMEGA..multidot.cm, an
electrification amount of 5 to 100 .mu.c/g of an absolute value,
and a particle size of 4 to 25 .mu.m is preferred. Further, a
material for the toner-equivalent material preferably is the same
materials as in the toner for development. That is, a material
comprising a styrene acrylic or polyester resin, carbon black as a
coloring agent and also a resistance controlling agent, silica as
an inorganic additive, and an electrification controlling agent as
main materials is preferred. A toner having a substantially
spherical shape and a particle size of 5 to 10 .mu.m prepared by
polymerization method using those main materials is particularly
preferred from the point of clearness (visibility) of image.
It is basically preferred for the covering layer comprising the
toner or the toner-equivalent material to decrease the thickness
thereof, but a thickness of 4 to 50 .mu.m is preferred from the
standpoints of adhesion to the toner for development and the
formation of electric field. The method of forming the covering
layer is not particularly limited, and any method can be employed
so long as it makes the surface of the developing roller smooth.
The formation can be conducted by, for example, spraying, dipping,
roll coating, brush coating, or the like. In particular, when the
toner-equivalent material is used as a material for covering layer,
the formation of the covering layer may be conducted such that the
developing roller is rotated in the toner reservoir at appropriate
times in the state that the elastomer layer having the surface
tackiness is exposed, thereby adhering the toner in the toner
reservoir to the surface of the covering layer.
By using the toner itself or a material having electrical
characteristics and mechanical characteristics equivalent to those
of the toner, specifically having the equivalent electrification
rank, as a material for the covering layer, the toner which is in
contact with the surface of the developing roller is not likely to
be electrified additionally. Therefore, the electrification
polarity and the amount of electrification of the toner are mainly
specified by the direction and intensity of an electric field
applied to the nearest region of the toner supplying roller and the
developing roller, and also the direction and intensity of an
electric field applied to a toner thin film formation region by the
blade 36. Control of those electric fields is easy, so that an
electrification state of the toner becomes stable.
The developing apparatus according to the present invention has a
very small scatter in electrification characteristics of the toner
as compared with the conventional developing apparatus, and
occurrence of the toner having an opposite polarity is minimized.
As a result, a very stable image can be maintained.
The developing roller of the present invention does not require a
resin-made protective layer which has conventionally been
indispensable, thereby being capable of reducing the cost. Further,
it is not necessary to select a material for a protective layer
satisfying the required characteristics such as
triboelectrification, and the selection of a material for the
developing roller can be made with only taking roller hardness and
electrical characteristics such as roller resistance into
consideration.
In the constitution of the present invention, the toner which
constitutes the covering layer also has a function as a protective
layer, and therefore does not contaminate the photosensitive member
(drum) even if a plasticizer and the like contained in the resin
composition cause bleeding. In particular, when an oxyalkylene
resin composition is used as a reactive organic material, the
oxyalkylene resin composition does not contain a plasticizer which
causes bleeding, and the amount of an unreacted product having the
possibility of bleeding is very slight, thereby making it difficult
to cause contamination of the photosensitive member (drum). As a
result, use of the oxyalkylene resin composition is preferred from
the standpoint of a long life of the developing roller.
The resin composition which is the constituent material of the
elastomer layer arranged on the shaft is explained below.
The resin composition constituting the elastomer layer, which can
be used, is a resin material having a conductivity or a
semiconductivity and containing a reactive organic material. This
resin composition is hereafter referred to as a "conductive
composition".
The conductive composition comprises an oxyalkylene, a saturated
hydrocarbon, an urethane, or a siloxane as a main component, and
includes compositions containing a reactive organic material which
becomes a solid from a liquid material upon a curing reaction. Of
those compositions, a composition comprising an oxyalkylene is
preferred for the reasons that it is easy to set the composition at
a low hardness, the volume resistivity of the material itself is
small, making it easy to impart the conductivity, viscosity of the
resin prior to curing is small, making it easy to process, and it
is difficult to contaminate the photosensitive member (drum).
Further, the composition comprising saturated hydrocarbon is also
preferred from the points that resistance when formed into a roller
is 10.sup.3 to 10.sup.8 .OMEGA. and resistance variation to the
atmosphere is small because water absorption is small. The curing
reaction includes a reaction due to the presence of an isocyanto
group, such as an urethane formation reaction or an urea formation
reaction, a hydrosilylation reaction, and hydrolysis condensation
reaction. Of those reaction, the hydrosilylation reaction is
preferred in that curing shrinkage does not substantially occur and
the curing time is short. Hereinafter, the composition comprising
the oxyalkylene is referred to as a "conductive composition (1)",
and the composition comprising the saturated hydrocarbon is
referred to as a "conductive composition (2)".
Use of the oxyalkylene type reactive organic material is explained
below.
The characteristics of this material are that it is easy to set the
material to a low hardness, volume resistivity of the material
itself is small, making it possible to impart conductivity,
viscosity of the resin prior to curing is small, making it easy to
process, and it is difficult to contaminate the photosensitive
member (drum), as described before.
The preferred example of the oxyalkylene type conductive
composition (1) is a conductive composition containing:
(A-1): a polymer having at least one alkenyl group in the molecule,
wherein a repeating unit which constitutes the main chain comprises
an oxyalkylene,
(B): a compound having at least two hydrosilyl groups in the
molecule (curing agent),
(C): a hydrosilylation catalyst, and
(D) a conductivity-imparting agent.
If the conductive composition to be obtained has a volume
resistivity of a semiconductivity region, component (D) may not be
used.
Component (A-1) is a component which cures by reacting with the
component (B). The component (A-1) has at least one alkenyl group
in the molecule. Therefore, hydrosilylation occurs to increase the
molecular weight, resulting in curing.
The number of the alkenyl group contained in the component (A-1) is
at least one from the point of hydrosilylation reaction of the same
with the component (B). However, considering a rubber elasticity,
it is preferred that two alkenyl groups are present at both ends of
the molecule in case of a linear molecule, and at least two alkenyl
groups are present at the ends of the molecule in case of a
branched molecule.
The repeating unit which constitutes the main chain of the
component (A-1) is an oxyalkylene unit. This is preferred in that
the volume resistivity of 10.sup.8 to 10.sup.9 .OMEGA..multidot.cm
can be obtained by addition of a small amount of the component
(D).
The composition of the component (A-1) is preferably that the
repeating unit which constitutes the main chain is an oxypropylene
unit from the standpoint of low hardness of a cured product.
The molecule of the component (A-1) wherein the repeating unit
which constitutes the main chain is an oxyalkylene has a number
average molecular weight (Mn) of 500 to 50,000, preferably 1,000 to
40,000, and more preferably 5,000 to 40,000. If the number average
molecular weight is less than 500, a sufficient mechanical
properties (such as rubber hardness, elongation, etc.) may not be
obtained when the conductive composition (1) of the present
invention is cured. On the other hand, if the number average
molecular weight is larger than 50,000, the molecular weight per
one alkenyl group contained in the molecule may increase, or
reactivity may decrease by a steric hindrance, resulting in
insufficient curing. In addition, there is a tendency that the
viscosity increases so that processability becomes poor.
The oxyalkylene type polymer, preferably oxypropylene type polymer,
means a polymer that at least 30%, and preferably at least 50%, of
the units constituting the main chain comprise the oxyalkylene
units. Units contained other than the oxyalkylene unit are units
derived from a compound having at least two active hydrogens used
as a starting material in the production of the polymer, such as
ethylene glycol, bisphenol compound, glycerine, trimethylol propane
or pentaerythritol. When the oxyalkylene polymer is the
oxypropylene polymer, the polymer may be a copolymer (including a
graft copolymer) of the oxypropylene with a unit such as ethylene
oxide, butylene oxide, or the like.
The alkenyl group contained in the polyoxyalkylene polymer is not
particularly limited, but the alkenyl group represented by the
formula
wherein R.sup.5 represents hydrogen atom or methyl group, is
preferred from the point of excellent curability.
One of the characteristics of the conductive composition (1) is
that it is easy to set the composition so as to have low hardness.
In order to exhibit this characteristic, the number of the alkenyl
groups is at least two at the ends of the molecule. If the number
of the alkenyl groups is too large as compared with the molecular
weight of the component (A-1), the composition is rigid, and it is
difficult to obtain good rubber elasticity.
Representative examples of the component (A-1) are compounds
represented by the following formulae (2) to (5). ##STR1##
The oxyalkylene type polymer prior to introducing the alkenyl group
can be obtained by conventional polymerization method of alkylene
oxide (anion polymerization method using sodium hydroxide) or chain
extension reaction using this polymer as a raw material. Further,
the oxyalkylene type polymer having a high molecular weight and a
narrow molecular weight distribution and containing functional
groups can be obtained by the methods described in, for example,
JP-A-61-197631, 61-215622, 61-215623 and 61-218632, and
JP-B-46-27250 and 59-15336.
The component (B) is not particularly limited so long as being a
compound having at least two hydrosilyl groups in the molecule.
However, if the number of the hydrosilyl group contained in the
molecule is too large, a large amount of the hydrosilyl group tends
to remain in the cured product even after curing, resulting in
occurrence of voids and cracks. Therefore, the number of the
hydrosilyl group contained in the molecule is 50 or less. From the
standpoints of control of rubber elasticity of the cured product
and the shelf life of the cured product, the number of the
hydrosilyl group is preferably 2 to 30, and more preferably 2 to
20. Further, from the standpoint of easy prevention of foaming at
curing, the number of the hydrosilyl group is 20 or less, and from
the point that poor curing is difficult to occur even if the
hydrosilyl group is deactivated, the number of the hydrosilyl group
is preferably 3. In view of the above, the most preferred number of
the hydrosilyl group is 3 to 20.
Having one hydrosilyl group means to have one SiH. Therefore,
SiH.sub.2 means to have two hydrosilyl groups. However, good
curability is obtained when hydrogen atoms bonded to Si are bonded
to different Si, rather than the same Si, and such is preferred
from the point of a rubber elasticity.
The component (B) has a number average molecular weight (Mn) of
preferably 30,000 or less, more preferably 20,000 or less, and most
preferably 15,000 or less, from the points of dispersibility of the
conductivity-imparting substance (the component (D)) and the roller
processability. Further considering the reactivity and the
compatibility with the component (A-1), the number average
molecular weight is preferably 300 to 10,000.
Specific examples of the component (B) are compounds having
hydrosilyl group-containing cyclosiloxanes at both ends of the
molecule, as shown in the following formula (6). ##STR2## wherein n
is an integer of 5 to 12, and m is an integer of 2 to 4, ##STR3##
wherein m is an integer of 2 to 4, ##STR4## wherein m is an integer
of 2 to 4.
In the formula (6), hydrosilyl group-containing cyclosiloxanes are
present at both ends of a relatively low molecule, but there is no
problem even if the hydrosilyl group-containing cyclosiloxanes are
also present at the ends of the polymer or a branched polymer.
Examples of the component (B) also include chain or cyclic
polyorganohydrogensiloxanes (including polyoxyalkylene modified
product, styrene modified product, olefin modified product, and the
like), as shown in the following formulae (7), (8) and (9).
##STR5##
A cohesive force of the component (A-1) is larger than that of the
component (B). Therefore, a phenyl group-containing modification is
important for the component (B) in the point of compatibility.
Styrene modified product is preferred from the points of
compatibility with the component (A-1) and easy availability and
denatured a-methyl styrene is preferred from the points of the
shelf life.
The hydrosilylation catalyst of the component (C) is not
particularly limited so long as it can be used as a hydrosilylation
catalyst. Examples of the hydrosilylation catalyst which can be
used include a platinum such as solid platinum supported on a
carrier such as alumina, chloroplatinic acid (including a complex
of an alcohol, or the like), various complexes of platinum, and
chlorides of metal such as rhodium, ruthenium, iron, aluminum or
titanium. Of those, chloroplatinic acid, a platinum-olefin complex
and a platinum-vinylsiloxane complex are preferred from the point
of catalyst activity.
Those catalysts may be used alone or as mixtures of two or more
thereof.
The amounts of the component (A-1) and the component (B) used are
such that the amount of the hydrosilyl group in the component (B)
is 0.2 to 5.0 mols, and preferably 0.4 to 2.5 mols, per mole of the
alkenyl group in the component (A-1) from the point of a rubber
elasticity.
The amount of the component (C) used is 10.sup.-1 to 10.sup.-8 mol,
preferably 10.sup.-1 to 10.sup.-6, and more preferably 10.sup.-3 to
10.sup.-6, per mole of the alkenyl group in the component (A-1). If
the amount of the component (C) used is less than 10.sup.-8 mol,
the reaction does not proceed. Further, the hydrosilylation
catalyst generally is expensive and corrosive, and also has a
property that a large amount of hydrogen gas generates, foaming the
cured product. Therefore, it is preferred that the component (C) be
used in an amount not exceeding 10.sup.-1 mol.
The conductive composition (1) of the present invention may contain
the conductivity-imparting substance as the component (D) in order
to obtain the desired roller resistance. However, if the roller
resistance to be obtained is relatively high, the
conductivity-imparting substance may not be added to the conductive
composition (1).
The conductivity-imparting substance as the component (D) is a
compound which can impart the conductivity to the composition.
Examples thereof include carbon black, metal fine powder, organic
compound or polymer each having quaternary ammonium salt group,
carboxylic acid group, sulfonic acid group, sulfonic ester group,
or phosphoric ester group, ether ester amide polymer, ether imide
polymer, ethylene oxide-epihalohydrin copolymer, a compound having
a conductive unit such as methoxypolyethylene glycol acrylate, and
an antistatic agent such as a polymeric compound. Those
conductivity-imparting substances may be used alone or as mixtures
of two or more thereof.
When the conductive-imparting substance as the component (D) is
added, the amount thereof is 30% by weight or less based on the
total weight of the components (A-1), (B) and (C), so that the
cured product has a volume resistivity of 10.sup.3 to 10.sup.10
.OMEGA..multidot.cm.
The embodiment of using the saturated hydrocarbon type reactive
organic material is explained below.
Preferred example of the saturated hydrocarbon type conductive
composition (2) is a composition comprising
(A-2): Polymer having at least one alkenyl group in the molecule,
wherein a repeating unit which constitutes the main chain is
saturated hydrocarbon,
(B): Compound having at least two hydrosilyl groups in the molecule
(curing agent),
(C): Hydrosilylation catalyst, and
(D): Conductivity-imparting substance.
Of the above components, the components (B), (C) and (D) are
substantially the same as in those in the conductive composition
(1) of the present invention. Therefore, explanation thereof is
given to only the different portion.
Similar to the component (A-1), the component (A-2) is a component
which cures by hydrosilylation reaction with the component (B).
Since the component (A-2) has at least one alkenyl group in the
molecule, hydrosilylation reaction occurs to increase the molecular
weight, resulting in curing.
The number of the alkenyl group contained in the component (A-2)
should be at least one from the point of hydrosilylation reaction
with the component (B). From the point of rubber elasticity, it is
preferred that two alkenyl groups are present at both ends of the
molecule in the case of a linear molecule. It is also preferred in
the case of a branched molecule that at least two alkenyl groups
are present at the ends of the molecule.
The repeating unit which constitutes the main chain of the
component (A-2) is a saturated hydrocarbon. Examples of the
component (A-2) are an isobutylene polymer, a hydrogenated isoprene
polymer and a hydrogenated butadiene polymer. Those polymers may
contain a repeating unit of other component such as copolymer, but
it is important for the component (A-2) that the amount of the
saturated hydrocarbon unit is 50% or more, preferably 70% or more,
and more preferably 90% or more, in that the characteristic of
small water absorption of the saturated hydrocarbon type polymer is
not impaired.
The component (A-2) has a number average molecular weight (Mn) of
about 500 to 50,000, and preferably about 1,000 to 15,000, from the
point of easy handling. The polymer which is liquid and has a
flowability at normal temperature is preferred from the point of
the processability.
A method of introducing the alkenyl group into the component (A-2)
is not particularly limited, and the alkenyl group may be
introduced during the polymerization or after the
polymerization.
Preferred examples of the polymer having at least one alkenyl group
in the molecule, wherein the repeating unit which constitutes the
main chain is hydrocarbon, are a polyisobutylene polymer, a
hydrogenated polybutadiene polymer and a hydrogenated
polyisobutylene polymer, each being a linear polymer having one
alkenyl group at the respective ends thereof and having a number
average molecular weight (Mn) of 2,000 to 15,000 and Mw (a weight
average molecular weight) /Mn of 1.1 to 1.2.
The compound (curing agent) having at least two hydrosilyl groups
in the molecule, which is the component (B) used in the conductive
composition of the present invention, is a component which
functions as a curing agent of the component (A-2).
The components (B), (C) and (D) which can be used in the conductive
composition (2) of the present invention are the same materials as
in the components (B), (C) and (D) in the conductive composition
(1).
The oxyalkylene type conductive composition and the saturated
hydrocarbon type conductive composition are briefly described above
as the preferred example of the resin composition as a material for
forming an elastomer layer. The details thereof are also disclosed
in U.S. Pat. No. 5,409,995, and JP-A-3-95266 and 6-256634.
The representative example of the conductive composition (1) or (2)
comprising components (A-1) to (D) or components (A-2) to (D),
according to the present invention is a conductive composition
comprising:
Component (A): a polyisobutylene, a hydrogenated polybutadiene, a
hydrogenated polyisoprene or a polyoxypropylene, each having a
number average molecular weight (Mn) of 2,000 to 15,000 and having
two alkenyl groups at the ends thereof;
Component (B): a hydrocarbon type curing agent having cyclic
hydrogenpolysiloxanes at both ends thereof or a chain or cyclic
polyorganohydrogensiloxane (a part of which may be modified with
styrene), in an amount such that the amount of SiH group is 0.7 to
1.4 moles, per mole of the alkenyl group in the component (A),
Component (C): 10.sup.-3 to 10.sup.-6 mol, per mole of the alkenyl
group in the component (A), of chloroplatinic acid (H.sub.2
PtCl.sub.2) or an alcohol solution thereof, as the hydrosilylation
catalyst,
Component (D): Ketjen black EC or acetylene black in an amount of 0
to 15% by weight based on the total weight of the components (A),
(B) and (C).
If required and necessary, the composition comprising the
above-described components (A), (B), (C) and (D) may contain a
shelf life improver as a component (E) for the purpose of improving
the shelf life.
Any material known as the shelf life improver for the component (B)
can be used as the component (E) if it achieves the desired
object.
Preferred examples of the component (E) include a compound having
an aliphatic unsaturated bond, an organophosphorus compound, an
organic sulfur compound, a nitrogen-containing compound, a tin
compound and an organic peroxide. Specific examples thereof include
benzothiazole, thiazole, dimethylmaleate, dimethylacetylene
decarboxylate, 2-pentene nitrile, 2,3-dichloropropene and
quinoline. However, the component (E) is not limited to those
materials. Of those, thiazole, benzothiazole and dimethylmaleate
are particularly preferred from the point of both the pot life and
the rapid curability.
The shelf life improver may be used alone or as mixtures of two or
more thereof.
The amount of the component (E) used can optionally be selected so
long as the component (E) uniformly disperses in the component (A)
and the component (B), but the amount thereof preferably is
10.sup.-6 to 10.sup.-1 mole per mole of the Si-H group-containing
compound as the component (B). If the amount of the component (E)
used is less than 10.sup.-6 mole, the shelf life of the component
(B) is not sufficiently improved, and if the amount thereof exceeds
10.sup.-1 mole, curing may be impaired.
The conductive composition (1) or (2), optionally containing the
shelf life improver, according to the present invention cures by
the reaction of the alkenyl group with the hydrosilyl group to form
a a rapid-curable cured product having a small volatile content.
Further, since the cured product of the conductive composition (1)
of the present invention contains the oxypropylene type polymer
(A-1) as the main component (in an amount of 60 to 98% by weight,
and preferably 90 to 97% by weight, based on the weight of the
cured product), the conductivity or semiconductivity of about
10.sup.3 to 10.sup.9 .OMEGA..multidot.cm can be obtained even by
addition of the component (D) in a small amount, and the conductive
composition having the characteristic that a rubber elasticity
including low temperature characteristics is good can be
obtained.
If required and necessary, the composition of the present invention
prepared from the above-described components (A) to (D), and
optionally the component (E), may contain a tackifying resin for
the purpose of increasing the surface tackiness of the elastomer
layer, a filler, an antioxidant, a plasticizer, an ultraviolet
absorber, a pigment, a surfactant, or the like.
Examples of the tackifying resin include a rosin ester resin, a
terpene-phenol resin, an aromatic petroleum resin, a xylene resin,
a phenolic resin, and a phenol-modified resin. It is preferred to
use the resin having a good compatibility with the reactive organic
material.
Examples of the filler include silica fine powder, magnesium
carbonate, clay, talc, titanium oxide, zinc white, diatomaceous
earth, and barium sulfate.
The tackiness of the surface of the elastomer layer necessary for
the developing roller of the present invention can be controlled by
changing the type and the amount of the curing agent in the case of
the conductive compositions (1) and (2).
The conductive composition (1) or (2) comprising the
above-described components (A) to (D) according to the present
invention is introduced in to a mold having a shaft made of SUS
arranged therein at the center thereof by cast molding, injection
molding or extrusion molding, and molded by heat curing at a
temperature of about 30.degree. to 150.degree. C., and preferably
80.degree. to 140.degree. C. for from 10 seconds to 1 hour, and
preferably 1 to 20 minutes. After semi-curing, the molded product
may be post-cured.
In forming the elastomer layer using the conductive composition, it
is important for the developing roller to have the nip width of a
certain width or more to the respective control blade and the
photosensitive member when the completed developing roller is
incorporated into the developing apparatus. The conductive
composition has a hardness of 40.degree. or less, and preferably
30.degree. or less, as defined by JIS Hardness A from the
standpoint of having such a nip width. The component (A) and the
component (B) are used in amounts such that the amount of the
hydrosilyl group in the component (B) is 0.2 to 2.5 moles, and
preferably 0.4 to 2.5 moles, per mole of the alkenyl group in the
component (A).
Of the materials which can be used as the resin composition which
is the material for forming the elastomer layer of the present
invention, the embodiment of using an urethane type resin
composition as the reactive organic material is explained
below.
The preferred example of the conductive composition (3) of the
present invention using the urethane type reactive organic material
is, for example, a conductive composition comprising:
(F): a polymer having at least one active hydrogen group in the
molecule, wherein the repeating unit which constitutes the main
chain comprises oxyalkylene or polyester,
(G): a compound having at least two isocyanato groups in the
molecule,
(H): a polyurethane formation catalyst, and
(D): the conductivity-imparting substance.
If the conductive composition to be obtained has a volume
resistivity of a semiconductivity region, the component (D) may not
be added.
If required and necessary, the conductive composition may contain a
plasticizer, and also the additives as described above.
The component (F) is a component which cures by reacting with the
component (G). Since the component (F) has at least one active
hydrogen group in the molecule, a polyurethane formation reaction
occurs to increase the molecular weight, resulting in curing.
The number of the active hydrogen group contained in the component
(F) should be at least one, and preferably 2 to 5, from the point
of the polyurethane formation reaction with the component (G). From
the point of the rubber elasticity, it is preferred that the active
hydrogen groups be present at both ends of the molecule.
The active hydrogen group may be an active hydrogen group formed by
only active hydrogen, or may be an active hydrogen group which is
present in the form of hydroxyl group, amino group or carboxyl
group. Although not particularly limited, it is preferred that the
active hydrogen group be present in the form of hydroxyl group from
the point of easy availability. Further, if it is desired to
decrease the tackiness of the cured product, it is preferred that
the active hydrogen group be present in the form of amino
group.
The repeating unit which constitutes the main chain of the
component (F) is preferably formed from the oxyalkylene unit or the
ester unit from the point of low hardness of the cured product. The
oxyalkylene unit, particularly oxypropylene unit, is more preferred
from the low hardness.
Specific example of the component (F) containing an active hydrogen
group as hydroxyl group, wherein the repeating unit of the main
chain is the oxyalkylene unit, is a polyoxyalkylene polyol obtained
by polymerizing an alkylene oxide having 2 to 4 carbon atoms with a
compound (starting material) having at least two active hydrogens,
used in producing the component (A-1). The polyoxyalkylene polyol
preferably used to produce the component (A-1) is also preferably
used as the component (F) of the conductive composition (3) of the
present invention.
Specific examples of the component (F) containing the active
hydrogen group, wherein the repeating unit of the main chain is the
ester unit, include polylactones such as ring-opening polymer of
.epsilon.-caprolactone, and polycondensates of polyoxyalkylene
polyol, dicarboxylic acid and low molecular weight diol.
Polybutadienes, hydrogenated polybutadienes, and polyols
(polyolefin polyols) such as polyisoprene can also be used in place
of the component (F) or together with the component (F).
Specific examples of the compound having at least two isocyanato
groups in the molecule as the component (G) include toluene
diisocyanate (TDI), diphenylmethane diisocyanate (MDI), crude or
polymeric MDI, hexamethylene diisocyanate (HDI), xylylene
diisocyanate (XDI), hydrogenated xylene diisocyanate (H.sub.6 XDI),
isophorone diisocyanate (IPDI), tetramethylxylene diisocyanate
(TMXDI), hydrogenated diphenylmethane diisocyanate (H.sub.12 MDI),
and derivatives obtained by prepolymerizing those compounds. Of
those, aliphatic diisocyanate compounds such as hexamethylene
diisocyanate are preferred from the point of low hardness. Those
compounds may be used alone or as mixtures of two or more
thereof.
The component (G) as the curing agent and the component (F) are
used in amounts such that the equivalent ratio of isocyanato group
in the component (G)/hydroxyl group in the component (F) is 0.7/1.0
to 2.0/1.0, and preferably 0.9/1.0 to 1.5/1.0, considering
unstability of the isocyanato group.
Specific examples of the polyurethane-formation catalyst as the
component (H) include organic tin compounds and tertiary amines.
which are conventionally used. Those compounds are generally used
in an amount of about 0.01 to 1% by weigh based on the total weight
of the component (F) and the component (G).
The component (D) is the same as the component (D) used in, for
example, the conductive composition (1) of the present invention,
and therefore the explanation thereof is omitted.
The plasticizer which can be added to the conductive composition
(3) of the present invention is used to decrease the hardness of
the cured product. However, when the plasticizer is added, bleeding
tends to occur as compared with no addition of the plasticizer.
It is preferred to add a large amount of the plasticizer from the
point of low hardness, but it is preferred to add a small amount of
the plasticizer from the point of decreasing the bleeding. The
amount of the plasticizer added is generally about 3 to 10% by
weight based on the total weight of the components (F), (G), (H)
and (D).
Specific examples of the plasticizer include phthalic acid
plasticizers such as DOP or DBP, and polyether plasticizers such as
polypropylene glycol (PPG) or polyethylene glycol (PEG).
Production of the roller, and the like can be conducted according
to the cases of the conductive compositions (1) and (2) of the
present invention, and the explanation thereof is omitted.
The embodiment of using the siloxane type reactive organic material
as the reactive organic material is explained below.
The preferred examples of the conductive composition (4) of the
present invention using the siloxane type reactive organic material
are a conductive composition comprising:
(I): a two-part room temperature-vulcanizable (RTV) silicone
rubber,
(J): a curing agent,
(K): a curing catalyst, and
(D): the conductivity-imparting substance.
and a conductive composition comprising:
(L): one-part RTV silicone rubber,
(M): a curing agent,
(N): a curing catalyst, and
(D): the conductivity-imparting substance.
If the conductive composition to be obtained has a volume
resistivity in a semiconductivity region, the component (D) may not
be added.
The conductive composition comprising the components (I) to (K) and
(D) has a good depth vulcanizability wherein the surface and the
inside are uniformly cured, and a good release property, and on the
other hand, the conductive composition comprising the components
(L) to (N) and (D) has a good adhesiveness. Therefore, a
composition having the combined characteristics of those two kinds
of the conductive compositions is preferred, and to this effect, a
mixture of those conductive compositions may be used.
When the tow-part RTV silicone rubber as the component (I) is an
addition type curable rubber, an alkenyl group such as vinyl group
is present in the molecule. This group reacts with hydrosilyl group
present in the curing agent to cause curing similar to the cases of
the conductive compositions (1) and (2) of the present
invention.
The two-part RTV silicone rubber used is a silicone rubber
conventionally used, and the curing agent therefor is, for example,
a siloxane type compound in the component (B) which is the curing
agent for the conductive composition of the present invention. The
curing catalyst used here is the same as used in the conductive
compositions (1) and (2) of the present invention.
The one-part RTV silicone rubber used as the component (L) is a
rubber having a silanol group in the molecule, and the curing agent
(M) used is a compound having at least two hydrolyzable silyl
groups. The silanol group formed by hydrolysis of the hydrolyzable
silyl group contained in the curing agent and the silanol group in
the one-part RTV silicone rubber are dehydration condensated to
cure.
The one-part RTV silicone rubber and the curing agent therefor
which can be used are conventional ones. Examples of the curing
catalyst include organotin compounds such as dibutyltin laureate,
dibutyltin dimaleate, dioctyltin dilaurate, dioctyltin dimaleate or
tin octylate; phosphoric acid or phosphoric ester such as
phosphoric acid, monomethyl phosphate, monoethyl phosphate,
monobutyl phosphate, monooctyl phosphate, monodecyl phosphate,
dimethyl phosphate, diethyl phosphate, dibutyl phosphate, dioctyl
phosphate or didecyl phosphate; organotitanate compounds;
organoaluminum compounds; saturated or unsaturated polycarboxylic
acid or acid anhydrides such as maleic acid or acid anhydride
thereof; amines such as hexyl amine, di-2-ethylhexyl amine,
N,N-dimethyldodecyl amine or dodecyl amine; and reaction products
of those amines and acidic phosphoric ester. The mixtures thereof
have high activity and are excellent.
In order to adjust the characteristics and to decrease the cost,
the conductive composition (4) of the present invention may contain
fillers such as fumed silica, precipitated silica, hydrophobic
silica, carbon black, titanium dioxide, ferric oxide, aluminum
oxide, zinc oxide, quartz powder, diatomaceous earth, calcium
silicate, talc, bentonite, asbestos, glass fiber or organic fiber.
Those filler may be used alone or as mixtures of two or more
thereof. Further, additives except the fillers may be used alone or
as mixtures of two or more thereof.
The production of the roller can be conducted according to the
cases of the conductive compositions (1) and (2) of the present
invention, and the explanation thereof is omitted.
Compositions other than the conductive compositions (1) to (4) of
the present invention are compositions comprising nitrile butadiene
rubber (NBR), styrene butadiene rubber (SBR), chloroprene rubber
(CR) or an ethylene-propylene rubber such as EPDM or an millable
silicone rubber, and a conductivity-imparting substance added
thereto, and compositions comprising the above compositions and
additives further added thereto. Those compositions are
thermoplastic, and therefore differ from the above-described
thermosetting compositions which form rubbery materials after
curing. Of the above rubbers, NBR and EPDM are preferred in having
a good balance between hardness and compression set when using as
the roller.
The millable silicone rubber is a heat-curable rubber obtained by
using a linear polyorganosiloxane (raw rubber) having a high degree
of polymerization (6,000 to 10,000) as the main raw material,
compounding a reinforcing or extending silica type filler as a
dispersion accelerator, and also various additives such as a heat
resistance improver, an inner release agent or a pigment with the
raw rubber to prepare a rubber compound, adding an organic peroxide
type curing agent to the rubber compound, and kneading the
resulting mixture. In general, the raw rubber contains
methylvinylsiloxane units.
The present invention is described in more detail by reference to
the following examples, but it should be understood that the
invention is not construed as being limited thereto.
EXAMPLE 1
100 g of a polyoxypropylene polymer (A-1) (90% of OH at both ends
of the polyoxypropylene were allyetherified) having a number
average molecular weight (Mn) of 8,000, a molecular weight
distribution (Mw/Mn) of 2.0 measured by gel permeation
chromatography (GPC) and a viscosity of 130 P(at 20.degree. C.),
6.9 g of a polysiloxane type curing agent (B-i) having SiH value of
0.358 mole/100 g represented by the following formula (10) ##STR6##
0.06 g of a 10% isopropyl alcohol solution of chloroplatinic acid
(H.sub.2 PtCl.sub.6), and 0.5 g of Ketjen black EC were mixed, and
degassed under reduced pressure of 10 mmHg for 120 minutes.
The composition thus obtained was applied to the periphery of a
stainless steel shaft having a diameter of 12 mm, and cured in a
mold at 120.degree. C. for 30 minutes to produce a roller having a
3 mm thick elastomer layer on the periphery of the stainless steel
shaft.
The initial mold releasability was good. Further, the cured product
was sufficiently adhered onto the stainless steel shaft.
Both ends of the roller were placed on a V block-shape support, and
a polyester type pulverized toner as a one-component non-magnetic
developer was sprayed on the surface of the rubber elastomer layer
of the roller while rotating the roller. The thickness of the toner
layer was measured with an outer diameter measuring machine
(manufactured by Keyence Co.) and was found to be about 30 .mu.m.
The developing roller thus obtained was set at the predetermined
position of a laser printer, and image evaluation was performed. As
a result, a sufficient image density was obtained, and fogging
phenomenon did not occur. Thus, very good result could be obtained.
The image could be maintained even after 1,000 printed matters.
Using the above composition, a film having a thickness of about 50
.mu.m was prepared using an applicator under the same conditions as
above. When tackiness of the surface of the film was measured with
an inclined ball tack test (inclination 30.degree., temperature
23.degree. C.) according to JIS Z 0237, the grade was 5. Since the
tackiness when formed into a roller and the tackiness of a film
have the same values if the curing conditions are the same, the
tackiness evaluation of the film was employed in place of the
tackiness evaluation of the roller.
Further, a cylindrical sample (no shaft) having a diameter of 30 mm
and a height of 12.7 mm was prepared using the above composition,
and hardness thereof was measured according to JIS Hardness A. As a
result, the hardness was 13.degree..
EXAMPLE 2
A roller having a rubber elastomer layer was produced in the same
manner as in Example 1. A polyester type polymer toner as a
one-component non-magnetic developer was sprayed on the surface of
the elastomer layer of the roller, and a covering layer was formed
in the same manner as in Example 1. Both initial image and image
after 1,000 printed matters were good.
EXAMPLE 3
A roller was produced in the same manner as in Example 1 except
that the polysiloxane type curing agent was changed to 2.7 g of a
polysiloxane type curing agent having SiH value of 0.97 mole/100 g
represented by the following formula (11) ##STR7## and image
evaluation was made. As a result, very good results could be
obtained. Further, good image was maintained even after 1,000
printed matters. The initial mold releasability was good, the
surface tackiness was 2, and JIS Harness A was 21.degree..
EXAMPLE 4
A roller was produced in the same manner as in Example 1 except
that 60 g of YS POLYSTER S-145 (manufactured by Yasuhara Yushi
K.K.; terpene-phenol resin) were added to the rubber compound, and
evaluated. The initial image was very good, but density unevenness
occurred after about 1,000 printed matters, although fogging did
not occurred. In addition, the initial mold releasability in
producing the roller was slightly poor. The surface tackiness was
13, the JIS Hardness A was 11.degree., and the thickness of the
toner layer was about 35 .mu.m.
EXAMPLE 5
A roller was produced in the same manner as in Example 3 except
that 20 g of titanium oxide was added to the rubber compound, and
evaluated. The initial image had density unevenness, but the degree
of image did not change even after 1,000 printed matters. Further,
the initial mold releasability was good, the surface tackiness was
1, the JIS Hardness A was 23.degree., and the thickness of the
toner layer was about 20 .mu.m.
The results obtained in Examples 1 to 5 above are shown in
Table 1 below.
In the results shown in Table 1, Example 2 is the result where a
polyester type polymer toner was previously adhered, and other
Examples are the results where a polyester type pulverized toner
was previously adhered.
TABLE 1
__________________________________________________________________________
First printed One thousand-the JIS Thickness Example Surface matter
(initial) printed matter Initial mold Hardness of toner No.
tackiness Intensity Fogging Intensity Fogging releasability A
(.degree.) (.mu.m)
__________________________________________________________________________
1 5 very none very good none good 13 30 good 2 5 very none very
good none good 13 -- good 3 2 very none very good none good 21 --
good 4 13 very none slight none slightly good 11 35 good intensity
unevenness 5 1 slight none slight none good 23 20 intensity
intensity unevenness unevenness
__________________________________________________________________________
EXAMPLE 6
A roller was produced in the same manner as in Example 5 except
that the toner adhered was changed to the polyester type
polymerization method toner, and evaluated in the same manner as in
Example 5. The tackiness was 1. However, the image up to 1,000
printed matters did not show density unevenness and fogging, and
development was very good.
EXAMPLE 7
A roller was produced using a composition composed of 100 g of a
polyisobutylene polymer having number average molecular weight (Mn)
of 10,000 and having two terminal vinyl groups, 2.5 g of a curing
agent having SiH value of 0.97 mole/ 100 g (containing 1 g of
dimethyl maleate as a shelf life improver in 100 g of the curing
agent) represented by the following formula (12) ##STR8## 75 g of a
plasticizer PS-32 (paraffin type, manufactured by Idemitsu Kosan
Co.), 3 g of Ketjen black EC, and 0.06 g of a 10% isopropyl alcohol
solution of chloroplatinic acid (H.sub.2 PtCl.sub.6), and was
evaluated in the same manner as in Example 6. Image up to 1,000
printed matters was very good. Further, the initial mold
releasability was good, the surface tackiness was 2, and the JIS
Hardness A was 15.degree..
EXAMPLE 8
A roller was produced in the same manner as in Example 7 except
that a polyisobutylene polymer having a number average molecular
weight of 6,000 and having two terminal vinyl groups was used in
place of the polyisobutylene polymer having a number average
molecular weight of 10,000 and having two terminal vinyl groups,
the amount of the curing agent used was changed to 4.2 g, and the
plasticizer PS-32 was not used, and was evaluated in the same
manner as in Example 7. The image up to 1,000 printed matters was
very good. Further, the initial surface mold releasability was
good, the surface tackiness was 1, and the JIS Hardness A was
29.degree..
EXAMPLE 9
A roller was produced using a composition composed of 100 g of
C-4190 (the main chain is polyether, NCO content: 4.5%; viscosity
700 cS/75.degree. C.; manufactured by Nippon Polyurethane Co.),
12.9 g of 4,4'-methylene-bis-2-chloroaniline. and 0.8 g of Ketjen
black EC under the curing conditions of 100.degree. C. and 10
hours. The toner was adhered thereonto and evaluated, in the same
manners as in Example 6. The first printed matter showed good
image, but slight density unevenness was observed as compared with
, for example, Example 6. And, the density unevenness was observed
also after 1,000 printed matters. The initial mold releasability
was good, the surface tackiness was 1, and the JIS Hardness A was
78.degree..
EXAMPLE 10
A roller was produced in the same manner as in Example 9 except
that a composition composed of 100 parts of a prepolymer (350 P at
4 rpm, 23.degree. C.) terminated with isocyanato groups obtained by
reacting isophorone diisocyanate (IPDI) with both ends of PPG
having a number average molecular weight (Mn) of 8,000 (dibutyltin
laureate catalyst, 80.degree. C..times.2 hours), 25 parts of a
polyoxypropylene triol (PPT) having a number average molecular
weight (Mn) of 3,000, and a tin catalyst was cured at 80.degree. C.
for 4 hours. The roller thus obtained was evaluated. Both the first
and thousandth printed matters had a good image, but a sight
density unevenness was observed as compared with, for example,
Example 6. The initial mold releasability was good, the surface
tackiness was 2, and the JIS Hardness A was 32.degree..
EXAMPLE 11
A roller was formed using a composition composed of 100 g of
CHEMIGUM N683B (NBR type rubber having bonded acrylonitrile content
of 33% and a Mooney viscosity of 28 (ML-4-100.degree. C.))
manufactured by Good Year Tire Co., and 5 g of Ketjen black EC by
injection method. The initial mold releasability was good. The
roller thus obtained was evaluated in the same manner as in Example
6. Both the first and thousandth printed matters showed good
results as the same as in Example 9, but slight density unevenness
was observed as compared with, for example, Example 6. The surface
tackiness was 1 to 2, and the JIS Hardness A was 45.degree..
COMPARATIVE EXAMPLE 1
A protective layer having a thickness of about 50 .mu.m was
provided on the surface of the roller obtained in Example 1 using a
20% methanol solution of CM-8000 (copolymerization nylon,
manufactured by Toray Co.) The surface tackiness was 0 and the
toner was not adhered onto the surface. However, as a developing
roller utilizing electrostatic force, very good image was obtained
until 1,000 printed matters. Fogging was slightly observed even
through it was in the level that had no practical problem. In this
case, the number production steps was large, and the production
cost was increased as compared with the above-described
Examples.
COMPARATIVE EXAMPLE 2
A roller was produced in the same manner as in Example 4 except
that the amount of YS POLYSTAR-S-145 used was changed to 80 g, but
the roller could not be taken out of the mold. The surface
tackiness was 15, and the JIS Hardness A was 9.degree..
The results obtained in Examples 6 to 11 and Comparative Examples 1
to 2 are shown in Table 2 below.
TABLE 2
__________________________________________________________________________
one thousand-the JIS Thickness of Example Type of conductive
Adhesion of printed matter initial mold Hardness toner No.
composition Tackiness toner Intensity Fogging releasability A
(.degree.) (.mu.m)
__________________________________________________________________________
6 Curable oxypropylene type 1 very good very good none good 23 20 7
Curable isobutylene type 2 very good very good none good 15 -- 8
Curable isobutylene type 1 very good very good none good 29 -- 9
Curable urethane type 1 good slight none good 78 -- intensity 10
Curable urethane type 2 good none good 32 11 NBR type 1-2 good
unevenness none good 45 -- good good Comparative Curable
oxypropylene type + 0 not adhered very good slight good 13 --
Example 1 Nylon 15 -- -- -- Not taken out 9 -- Example 2 Curable
oxypropylene type
__________________________________________________________________________
It can be understood from the results shown in Tables 1 and 2 above
that when the surface tackiness of the rubber elastomer layer is 1
to 13, and preferably 2 to 13, in the inclined ball tack test
(inclination 30.degree., 23.degree. C.), the one-component
non-magnetic developer is well adhered, and the electrification and
the amount of the one-component nonmagnetic developer thereon can
be made uniform, thereby good image can be obtained.
On the other hand, it is understood that when the surface tackiness
is more than 13, the initial mold releasability becomes poor, and
when the surface tackiness is 15 or more, the roller cannot be
released from the mold.
The developing roller according to the present invention has the
following effects.
(1) Since the surface of the developing roller is covered with a
toner itself or a material having electrical and mechanical
characteristics substantially equal to those of the toner, the
chance that the developing toner contacts with foreign materials
can be minimized as less as possible. Therefore, the factors for
determining the electrification polarity and the electrification
amount of the toner can substantially be limited to only the
direction and strength of the electric field for electrification,
it is possible to make the electrification polarity and the
electrification amount of the toner stable, a sufficient image
density can be obtained without fogging, and very stable image can
be obtained. Further, such a useful developing roller can be
obtained by merely adhering the toner itself or a material having
electrical and mechanical characteristics substantially equal to
those of the toner on the surface of the developing roller
utilizing the surface tackiness of the developing roller.
Therefore, the production cost does not increase.
(2) The covering layer comprising a toner or a material having
substantially the same triboelectrification rank, which covers the
surface of the developing roller also exhibits the function as the
protective layer which prevents the conductive composition from
being directly contacted with the photosensitive member (drum).
Therefore, it is not necessary to arrange a protective layer made
of a resin, which requires cost for formation of such layer, and
there is no problem on the contamination of the photosensitive
member (drum).
(3) In the case of applying the developing roller to a color system
developing apparatus, although the electrification characteristics
of each color toner (C, Y, M, B) differ, the surface of the
developing roller arranged corresponding to each color toner is
covered with the same color toner or a material having
substantially the same electrical and mechanical characteristics as
the same color toner, so that color matching can be performed with
only control of electric field as the same as in monochrome, and a
stable color development can be conducted.
* * * * *